In a talk at the recent TedxManhattan conference, writer Frederick Kaufman described his attempts at understanding the “sustainability index,” a leading measure in corporate America of the sustainability of various foods.

The index seeks to label any given food with a single number, so that a consumer can make an informed choice about how “sustainable” his tomato or her Froot Loops might be. It covers a complex number of measures including the amount of resources, past and present pollution, environmental efforts, and society itself.

What Kaufman found comes as no surprise to those of us who recognize the shortcomings of sustainability. Sustainability, sustainable development, and sustainable growth have become popular terms, but their usage does not reflect the complexity of the terms’ meanings. While they bring consensus around nice-sounding ideas, these terms obscure underlying questions and conflict.

The goals of sustainability cannot be to sustain everything forever; that is nonsensical. Rather, it prescribes sustaining some things for some time period, which is clearly more reasonable. But, what does it preserve, and for how long? Should we seek to sustain stocks, such as the number of fish in the sea, or capacities, such as an ecosystem’s ability to rejuvenate after a forest fire? These are not easy questions.

However, the popular idea of sustainability provides deceptively easy answers. A sustainability index brings together assessments of a wide range of factors involved in the production of a particular good — i.e. water usage, carbon emissions, the loss of habitat, degradation of fish stocks, etc. — and assigns these factors a single value. This is a classic example of the fallacies of measurement that plague all sorts of ranking systems, from automobiles to colleges, as author Malcolm Gladwell has pointed out. Rankings fail when they try to be both comprehensive and heterogeneous.

In many ranking systems, and particularly in the measurement of sustainability, the various inputs are not commensurable. We cannot compare the value of different units' quantities; we cannot talk about the amount of water used, in terms of the number of trees cut down, in terms of carbon emitted, in terms of a fish population destroyed.

Yet a sustainability index seeks to do just that. It obscures the trade-offs implicit in the underlying processes of production and the crucial question of what is to be sustained.

Simple metrics can be helpful, even if they do not tell the whole story. Calorie counts give a useful baseline for understanding how healthy a given food is. The impulse to offer a similar guide to consumers for sustainability is a worthy one.

But, a simple metric for sustainability hides important trade-offs. These trade-offs should instead be brought into the open. Consumers, producers, and government regulators alike should be engaged in informed debate — either through political channels, or simply by voting with purchasing power — over which sustainability metrics are most important. This sort of debate can only happen if the messy complexity of sustainability is acknowledged and embraced, not obscured by a facile index.